ln10s08 - Lecture 10: Water Equilibria Part III-Solving the...

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Water Equilibria Part III -- Solving the weak acid and weak base cases. This is our third lecture on acid base water chemistry. In the first one we learned about the temperature dependent calculation of pH from Kw for pure water. In the second lecture we learned that strong electrolytes dissociate completely in water and if those strong electrolytes include strong acids or strong bases, it is easy to estimate a pH since the amount of proton or hydroxide in solution is simply the amount of strong acid or strong base that you added to solution initially. Today’s lecture looks at a third case in which you throw a weak acid or base into solution and predict the pH. Once again we will be using the RICE expression and will now derive the weak acid or weak base equations from it. Things will be a little more complicated and will require that we make a few assumptions in order to yield a really simple equation to find the pH of the solution. But in the end, we use the same theoretical foundation involving chemical equilibria from Chapter 9. Time out for nomenclature of weak acids and bases. There are an infinite number of weak acid or weak base compounds that we can draw, and it doesn’t take long before some of them look so complicated that we think we can’t even begin to solve the calculation. But what is most important to note is that weak acids and weak bases are based on the combination of two simple parts a conjugate base that is either charged or uncharged (A - or B) and is in all weak bases and acids a proton (H + ) that is attached to the weak base to form a weak acid. So no matter how complicated the structure, we will see that we can always write simple equilbria expressions that involve B A - BH + HA a neutral weak base a charged weak base a charged weak acid a neutral weak acid For this lecture we will work with the form with which you are most familiar: HA H + + A - that has K a = [H + ] [A - ]/[HA] We will see shortly that the RICE expression solution for this weak acid equilibria is simply
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ln10s08 - Lecture 10: Water Equilibria Part III-Solving the...

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